1. Field of the Invention
Present invention relates to an objective lens system for use in photography and more particularly to a lens system that is designed to accommodate a frontal aperture stop.
2. Brief Description of the Prior Art
In an ordinary photographic lens system, a diaphragm is generally interposed between the lenses within the lens assembly, for example as seen in U.S. Pat. No. 3,438,697. Generally the entrance and exit pupils will also be located within the lens assembly and are frequently defined by the lenses themselves. For a more detailed discussion of the design factors for determining the amount of light passing through an optical system and the elements that delineate the portion of the object field actually imaged, reference is made to "Optics: A Short Course for Engineers and Scientists" by Williams et al, Wiley-Interscience, 1972.
For certain photographic apparatus it is sometimes necessary to have an aperture stop provided in the front or the rear of the lens system. For example, when the lens assembly is to be built directly into the camera body, it is necessary, for economic reasons and/or structural design requirements of the camera body, to have a diaphragm, shutter and other mechanisms mounted in front of the lens assembly.
A problem in such a lens system is to provide a sufficiently high aperture ratio while maintaining a normal photographic field angle and finally providing satisfactory compensations for aberrations. The prior art has found it difficult to increase the diameter of the lenses in the rear components of the lens systems, and further a problem to establish a satisfactory balance between the spherical aberration and astigmatism particularly when the aperture ratio or lens opening is to be increased. As is well known, the size of the aperture determines the amount of volume of light reaching the film from a given subject. A high aperture ratio lens system with a frontal aperture stop providing a brightness of F2 or greater while maintaining a normal photographic field of angle of approximately 45.degree. with a satisfactory compensation for aberrations, has been difficult to achieve in the prior art.